We plan to study patients with suspected or confirmed PML with different underlying conditions including patients on immune-modulatory therapies for multiple sclerosis (MS), rheumatologic diseases or other autoimmune diseases, as well as patients with HIV infection or other conditions leading to a compromised immune system. Patients will be seen at defined time points during their disease course and detailed assessments will be performed to collect clinical and imaging data. Blood and cerebrospinal fluid (CSF) will also be collected at these time points to evaluate the behavior and biology of the JCV and the patients immune responses to the infection. These tests will lead to a better understanding of the pathophysiology of PML and the course of this disease in different patient groups.